How to detect that the fitness landscape of a genetic algorithm is changing over time?
There is no exact way to assess that a genetic algorithm has located a global optima. Indeed there may be multiple global optima. You must fall back to heuristic methods. The fitness of a population is the maximum fitness of any individual. Unless specific measures are taken to maintain diversity the population will converge to an optima, local or global. At that point all individuals will, except for mutation, be identical. You could take the fittest individual of such a population as your solution, but you will not know if the solution is a global or local optima.
Two reasonable heuristics are these. First, run the algorithm till it converges and maintains its fitness for a number of further generations. Or second run the algorithm multiple times, and take the fittest of all the located solutions. Neither is exact.